This worked for me as well. I have an LG V30 and was trying to set up Samsung Pay. In the Wear app I clicked "Open Samsung Pay" and a message popped up at the bottom of the screen saying "Installing Samsung Pay (wear plug-in)" then it would go away. I would then click "Open Samsung Pay" and this message would pop up again. This continued on a loop every time I would try to get samsung pay to work.

The solution is to install the APK manually downloaded from the internet. Everything functions fine now.

Ok, just to clarify, did you download it with your phone, or with your watch? I downloaded it with my phone, and got prompted that it wasn't from a trusted source-which I approved so that I can download it, but after openning it, it says that my watch may not be supported.